1. Field of the Invention
This invention relates to optical transceivers. In particular, this invention relates to a differential photoelectric receiver circuit having low noise and a wide bandwidth.
2. Background of the Invention
An infrared transceiver is a high gain, wide bandwidth system. The photoelectric sensor or front edge stage of the receiver, which converts optical energy into an electrical voltage, is critical to the operation of the transceiver. In order to design an effective photoelectric receiver many factors must be considered, two of the most important being noise and bandwidth.
Typically the front edge stage comprises a transimpedance amplifier for converting the photocurrent from a photodiode (PD) to an electrical voltage proportional to the optical intensity. For example, FIG. 1 illustrates a prior art photoelectric receiver transimpedance amplifier circuit in an infrared transceiver. The PIN diode PD which is connected between the bias power supply Vp and the inverting input of the operational amplifier U provides a current in proportion to the intensity of an infrared signal striking the active area of the PIN diode PD. The non-inverting input of the operational amplifier is grounded, creating a virtual ground on the inverting input.
This circuit has the advantage of providing a constant bias on the PIN diode PD due to the virtual ground. However, the photoelectric receiver circuit of FIG. 1 has poor immunity to noise and interference because it lacks a differential structure, and therefore cannot reject common mode noise and interference. Moreover, the bias voltage Vp must provide the full photocurrent, which can be as large as several tens of milliamperes, and it is very difficult to integrate into the same chip as the front edge circuit a voltage regulator which has low noise and yet provides such a large current. The noise of the voltage regulator would become the main source of noise in the system.
FIG. 2 illustrates a prior art differential photoelectric receiver. As long as R1=R2, this circuit is a differential circuit for small signals and thus provides the advantage that it has a very good common mode noise rejection ratio. The diode PD is AC coupled to the operational amplifier U through identical capacitors C1 and C2, as required in infrared communication to filter out low frequency optical noise, and the capacitors C1, C2 eliminate the effect of the PIN diode PD bias to the input bias of the differential amplifier U.
The disadvantages of the circuit of FIG. 2 are:
1) The PIN diode PD bias is not constant. When the photocurrent detected by the diode PD increases the PIN diode bias decreases, so that the parasitic capacitance of the PIN diode PD increases and the bandwidth of the diode PD decreases. Thus, when the optical power is very strong communication pulses tend to merge.
2) The bias voltage Vp must provide the full photocurrent, as in the circuit of FIG. 1, which presents the same problem of implementing the photoelectric receiver circuit of FIG. 2 with a voltage regulator into a single chip.
3) Since the resistances of R1 and R2 cannot be very large (the maximum allowable resistance would be in the order of 10 kW), the capacitance of the coupling capacitors C1, C2 must be large to reach a required low cutoff frequency and would occupy a substantial area of the chip die.
U.S. Pat. No. 5,389,778 issued Feb. 14, 1995 to Shinomiya teaches a photoelectric conversion circuit utilizing transimpedance amplifiers in association with frequency selecting components comprising a tuning circuit and LC resonators. The frequency selecting components form a narrow bandwidth network, which filters out noise outside of the selected band. However, this circuit is not symmetrical and therefore cannot reject common mode noise over a wide bandwidth. Further, the PIN diode bias is supplied by the voltage source directly, so that the photocurrent, including noise from the voltage source, flows through the circuit as a load current, resulting in poor common mode noise rejection.
U.S. Pat. No. 5,343,160 issued Aug. 30, 1994 to Taylor teaches a symmetrical transimpedance amplifier circuit for a photoelectric receiver, which utilizes a combination of DC feedback loops and AC feedback loops to control the gain of the circuit. The use of AC coupling in this case reduces the low frequency performance of the circuit, and AC feedback from the operational amplifier output supplies the PIN diode bias voltage, so that when the optical signal has a wide bandwidth, the low frequency components (which are prevalent in optical communication systems) can decrease the PIN diode bias voltage due to the low frequency gain of the AC signal coupling amplifier. This increases the PIN diode parasitic capacitance and causes degradation of the high frequency performance of the circuit as well as extra phase shifting, which can lead to instability.
The present invention thus provides a photoelectric receiver circuit for converting an optical signal to an electrical signal, comprising: first and second transimpedance amplifiers, a photodiode having a first end connected to an inverting input of the first transimpedance amplifier and a second end connected to an inverting input of the second transimpedance amplifier, and a differential amplifier having inputs AC coupled to outputs of the first and second transimpedance amplifiers, wherein when higher and lower voltages are respectively applied to the non-inverting inputs of the first and second transimpedance amplifiers, a substantially constant bias voltage is maintained on the photodiode.
The present invention further provides an optical transceiver comprising a photoelectric receiver circuit for converting an optical signal to an electrical signal, the photoelectric receiver circuit comprising: first and second transimpedance amplifiers, a photodiode having a first end connected to an inverting input of the first transimpedance amplifier and a second end connected to an inverting input of the second transimpedance amplifier, and a differential amplifier having inputs AC coupled to outputs of the first and second transimpedance amplifiers, wherein when higher and lower voltages are respectively applied to the non-inverting inputs of the first and second transimpedance amplifiers, a substantially constant bias voltage is maintained on the photodiode.